In a wide variety of applications in industry, chemical installations, metallurgical plants and, more generally, wherever thermal energy is generated and used, it is desirable to provide for the storage of such thermal energy when an excess is produced and for the subsequent tapping of the stored thermal energy.
For example, storage and recuperator furnaces or ovens have been provided heretofore in which bodies of high thermal capacity are heated, e.g. by a gas during periods of excess heat development and subsequently transfer the stored heat to a fluid at a lower temperature when consumption of the heat is desirable.
The heat-storage body can be composed of refractory material, e.g. fire clay, sillimanite, mullite, magnesite, chrome-magnesite, chrome ore, zirconia, whose densities and melting points increase in the given order.
These materials vary with respect to the significant parameters involved in their use as heat storage materials, namely, the specific heat (j.multidot.h/kg degree), thermal conductivity (W/m.multidot.degree) and the heat content (J/kg or J/degree.l).
Fire clay, for example, at 1200.degree. C. has approximately twice the specific heat of the zirconia while its thermal conductivity coefficient is substantially less than that of zirconia.
Magnesite has a specific heat which is comparable with that of fire clay but has three times greater thermal conductivity.
It is thus desirable in producing heat-storage bodies, e.g. bricks, of refractory material to obtain the best combination of properties at a reasonable or low cost.
In British Pat. No. 1,262,475, for example, refractory materials are described for the thermal storage cores in electrical storage heaters and the problem with low density refractory materials is discussed. As a solution to the problem, this patent proposes a heat-storage body which consists essentially 100% of Fe.sub.2 O.sub.3.
In this system, the oxide is compacted with a pressure which is upwards of 300 kg per cm.sup.2 and can be less than 800 kg per cm.sup.2 to yield a body of high thermal conductivity. In addition, the reference indicates that one can use Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4, the latter being oxidized to Fe.sub.2 O.sub.3. This oxidation can be effected during the sintering step which is applied after pressing.
The iron is thus in its highest oxidation state or is transformed into the highest oxidation state, eliminating difficulties with subsequent oxidation in operation with available oxygen which might tend to alter the physical chemical characteristics of the body. The trivalent iron thus forms a relatively stable product.
Swiss Pat. No. 453,626, on the other hand, discloses that the oxidation of Fe.sub.3 O.sub.4 at 750.degree. C. in air is surprisingly so minimal that the use of this material as a heat-storage body is recommended, the Fe.sub.3 O.sub.4 being cast into bricks.
Whether one choses to believe one or the other teaching, it is clear that unless the iron oxide is in the pure form, in which it can only be obtained at extremely high cost, any body which consists predominantly of iron oxide runs the risk of being susceptible to alterations in the physical chemical parameters during use and damage to the bricks or the like.